Two monitors on the same computer. The required computer hardware is either two display adapters or one display adapter with two ports. The display adapters are typically set to "extended desktop mode," in which both monitors function as one unit with the mouse moving seamlessly from one screen to the other. See window spanning and extended desktop mode.
The More the Merrier
The two 21" Samsung monitors were first connected to one PC (top). Later flipped into portrait mode, they became the smallest monitors in a mixed Windows/Mac configuration (bottom). The largest monitor is 30" from Apple, while the rightmost is a 26" Samsung.
Windows automatically detects multiple display adapters and lets you configure them individually. This Display Properties dialog box is displayed by right clicking the mouse on the desktop.
Dual Port Card
High-end display adapters such as this card from NVIDIA have two DVI ports for two LCD monitors. (Image courtesy of NVIDIA Corporation.)